May 27, 2009—For the first time, astronomers have mapped the phases of a planet outside our solar system (top)—just as Italian astronomer Galileo Galilei drew the phases of Venus (bottom) almost 400 years ago.

The extrasolar planet, or exoplanet, COROT-1b orbits a star about 1,600 light-years from Earth.

The planet is known as a "hot Jupiter," a class of exoplanets similar in size to Jupiter but orbiting very close to their host stars—usually less than an eighth of the distance between Mercury and the sun.

Astronomers had suspected that hot Jupiters would be what's known as tidally locked to their hosts—with one side always facing the star, just as one side of the moon always faces Earth.

Using the COROT satellite, Ignas Snellen of the Netherlands' Leiden University and colleagues collected visible light readings from COROT-1b over the course of 36 of its orbits.

The team observed a contrast in brightness between the two sides that creates phases as the planet orbits its star. These phases are similar to the phases of the planets Mercury and Venus as seen from Earth.

"This is a very exciting new result, in that astronomers have been searching for a decade to try and detect [visible] starlight reflected from the day sides of hot Jupiters," Andrew Collier Cameron, an astronomer at the University of St. Andrews in Scotland, commented via email.

But while Galileo used the phases of Venus as evidence that the planets orbit the sun, Snellen and colleague's findings confirm that COROT-1b is in fact tidally locked to its star, with one side of the planet in permanent darkness and the other illuminated all the time.

The team also took new measurements of COROT-1b's temperature, which showed that the planet experiences huge temperature differences, ranging from about 4,082°F (2,250°C) on the day side to 2,282°F (1,250°C) on the night side.

Not having a more balanced heat distribution indicates that the planet's atmosphere doesn't mix very fast, the study authors say. (Related: "Half-Hot, Half-Cold Planets Have Supersonic Jet Streams.")

"Wind speeds are probably not high enough to allow the energy to be transported from the day side to the night side before it is radiated back into space," said Ernst de Mooij, a study co-author.

Furthermore, the atmosphere must contain chemicals, such as titanium oxide, that rapidly absorb heat for it to reach the searing temperatures observed.